The use of the catalytic cracking process in the oil industry at the end of the thirties represented considerable progress with respect to the previous techniques, by providing for highly improved yields of motor gasoline of very high grade. The first processes operating in a fixed bed (e.g. HOUDRY process) were quickly surpassed by moving bed processes and mainly, since the middle of the forties, by the fluid bed processes (FCC or Fluid Catalytic Cracking). At the very beginning of their use, the catalytic cracking processes were almost exclusively devoted to the treatment of relatively light Vacuum Distillates (VD) of low sulfur content (final boiling point lower than 540.degree.-560.degree. C.).
The cracking of these charges is performed at about 500.degree. C. at a total pressure close to atmospheric pressure and in the absence of hydrogen. In these conditions the catalyst becomes quickly covered with coke deposits and its continuous regeneration is necessary. In the fluid bed or moving bed cracking processes (FCC or TCC) the catalyst continuously circulates between a reaction zone where its residence time is from about several seconds to a few tens of seconds and the regenerator where it is freed from coke by combustion at a temperature from about 600.degree. to 750.degree. C. in the presence of diluted oxygen. The fluid bed units (FCC) are now much more widely used than those of the moving bed type. In these units the catalyst circulates in fluidized state, as small particles of average diameter ranging from 50 .mu.m to 70 .mu.m, the particle size of said powder ranging approximately from 20 .mu.m to 100 .mu.m.
The catalysts used in the first FCC units were solids of high silicon content obtained by acid treatment of natural clays or synthetic silica-aluminas. Some of the main improvements achieved by FCC up to the end of the fifties have been:
The use of the spray-drying technique whereby the catalysts are obtained as fine spherical particles adapted to be more easily fluidized and more resistant to attrition than the powders obtained by crushing,
The manufacture of synthetic silica-aluminas, initially of high silica content "low alumina" or Lo-Al of about 85% by weight silica content, and then of higher alumina content ("High Alumina" or Hi-Al of about 75% by weight SiO.sub.2 content), and
Various very important improvements concerning metallurgy and design of equipments, particularly for regeneration.
But it is only at the beginning of the sixties that a major improvement was achieved in the field of catalytic cracking by the use of molecular sieves and more precisely of zeolites of the faujasite structure, first in a moving bed process, then, a little later, in FCC. These zeolites, incorporated to a matrix consisting mainly of amorphous silica-alumina, optionally containing variable proportions of clay, are characterized by cracking activities, with respect to hydrocarbons, of about 1000 to 10000 times those of the initial catalysts. The availability on the market of these new zeolite catalysts has resulted in a drastic change of the cracking process both by a very substantial increase of the activity level and of the selectivity to gasoline and by considerable changes in the unit technology, mainly:
cracking in the "riser" (tube wherethrough the catalyst and the charge flow upwardly, for example), PA1 decrease of the contact times, PA1 modification of the regeneration techniques. PA1 better thermal and hydrothermal stability and better tolerance to metals, PA1 lower coke formation at equal conversion rate, PA1 production of a gasoline of higher octane number, PA1 improved selectivity to middle distillates. PA1 a high thermal and hydrothermal stability, the interest of which has already been stated. PA1 a capacity to limit coke production. PA1 an excellent selectivity to middle distillates. PA1 (1) Removal of organic cations by roasting in air. PA1 (2) Exchange of alkali cations (Na.sup.+) with ammonium cations. PA1 (3) Roasting in the presence of steam. PA1 (4) Acid etching.
X zeolite (Faujasite structure characterized by a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio from 2 to 3) was the first used. Highly exchanged with rare-earth ions, it had a high activity and high thermal and hydrothermal resistance. Toward the end of the sixties, this zeolite was progressively replaced by Y zeolite, whose tendency to produce coke was slightly less and whose thermal and hydrothermal resistance was greatly improved. To date, in a large part, (probably more than 90%) the proposed catalysts contain a Y zeolite exchanged with rare-earths ions and/or ammonium ions.
From the beginning of the seventies, the oil industry suffered from a shortage of crude oil supply whereas the demand for gasoline of high octane number still increased. Moreover, heavier and heavier crude oils were progressively included in the available supply. The treatment of the latter was a difficult problem for the refiner in view of their high content of catalyst-poisoning substances, particularly nitrogenous compounds and metal compounds (mainly nickel and vanadium), unusual values of Conradson Carbon and mainly asphaltene compounds.
The required treatment of heavier charges and other more recent problems such as the progressive, but general, decrease of the proportion of lead-containing additives in gasoline, the low, but significant, evolution in various countries of the demand for middle distillates (kerosene and gas-oils), have induced refiners to undertake further searches with a view toward obtain catalysts having the following improved performances:
In view of the present tendency of the charges to produce more and more coke on the catalyst and of the high sensitivity to coke of the zeolite performances, it is now desirable, not only to search for catalysts less selective to coke but also to further the catalyst regeneration in order to reduce to a minimum the coke amount at the end of the combustion, and this requires, in certain processes, an increase of the regenerator temperature. Thus it happens that high steam partial pressures from 0.2 to 1 bar prevail in the regenerator with local temperatures, at the catalyst level, from 750.degree. to 850.degree. C. or even 900.degree. C., for a time from a few tens of seconds to a few minutes. In these conditions, the zeolite, which is the main active agent of the catalyst, may quickly lose a substantial part of its activity as a result of an irreversible degradation of its structure. In spite of the various technological tricks used during the last years to reduce the regenerator temperature (addition of coils to remove heat by producing steam or by intermediary cooling of the catalyst) or for reducing the steam content at high temperature (techniques with two regenerators as used in the R2R TOTAL-I.F.P. process) it is necessary that the zeolite present in the cracking catalyst exhibit an excellent thermal and hydrothermal stability.